1. Field of the Invention
This invention generally relates to lithography tools for the manufacture of microelectronic components, and more particularly, to a method and apparatus for analyzing image placement on a pre-distorted lithographic mask.
2. Description of Related Art
U.S. Pat. No. 5,308,991 of Kaplan for "Method and Apparatus for Making a Predistorted Reticle to Compensate for Lens Distortions" describes a method of generating a global pre-distortion for a mask, but does not discuss any method of evaluating the resulting distorted mask. Kaplan provides corrections only at locations addressed by the stage and not at locations reached by summing an electromagnetic deflection to the stage position. The metrology sites used to provide for disposition of the mask will typically be at locations reachable only through combined deflection.
U.S. Pat. No. 5,481,624 of Kamon for "Mask Inspecting Method and Mask Detector" describes a method of inspecting phase shift masks. Here an optical element is introduced into the inspection tool which perceives the mask images in a fashion similar to what would be exposed on the wafer. Since the goal of any mask technology, included phase shifting is to reproduce the design CAD data, this method would allow the user to determine if a phase shift mask is working, though it does not give the user complete information regarding problems that may be detected. This method works with the generation of a pre-distorted feature shape, but not feature location, The disclosure works with pre-distorted feature locations.
U.S. Pat. No. 4,559,603 of Yoshikawa for "Apparatus for Inspecting a Circuit Pattern Drawn on a Photomask Used in Manufacturing Large Scale Integrated Circuits" describes a method of enhancing the resolution of a mask inspection device. The reference CAD data is manipulated to generate a synthetic signal that matches the expected signal from the mask. A mask that is pre-distorted by a means other than the manipulation of the CAD data could not be inspected with this system.
U.S. Pat. No. 4,600,996 of Kawauchi for "Arrangement for Inspecting Circuit Patterns Drawn on a Mask" describes a method of comparing an exposed pattern to the design CAD data as stored in an E-beam exposure tools memory. To the extent that any intended pre-distortion is residing in that location in memory, then the Kawauchi method could in principle evaluate a pre-distorted mask. The Kawauchi method will not see pre-distortion that is introduced through a different channel, such as the laser response method outlined in the Kaplan '991 patent. In that case, the method of Kawauchi patent would treat the desired pre-distortion as an error.